The present invention relates to an automatic control apparatus for wind-shield wipers, and more particularly to an electro-optical liquid detector assembly for the automatic control apparatus which is adapted to electro-optically detect an adherence condition of liquid such as raindrops on the external surface of a wind-shield of wheeled vehicles, airplanes, ships or the like so as to produce an electric control signal necessary for automatic control of a wind-shield wiper in dependence upon a result of the detection.
In a Japanese Early Publication for Utility Model No. 51-5141, there has been proposed an electro-optical detector assembly which comprises a reflection plate attached to the internal surface of a wind-shield at a portion where the external surface of the wind-shield is wiped by a wiper blade, a pair of spaced prisms attached to the internal surface of the wind-shield at opposite sides of the reflection plate, a light emitting element arranged to emit a beam of light into the wind-shield through one of the prisms so as to cause multiple internal reflection of the beam between the external surface of the wind-shield and the reflection plate, and a light receiving element arranged to receive the incident beam reflected by the external surface of the wind-shield through the other prism. In such a conventional liquid detector assembly, the prisms each are, in general, made of transparent synthetic resin such as poly-carbonate, acrylate or the like. In making of such a prism of transparent synthetic resin, a time difference will occur in coagulation of the respective portions of the prism. For this reason, the prism is formed thereon with uneven surfaces and therein with air bubbles. Such uneven surfaces on the prism and/or air bubbles in the prism will cause turbulent reflection and irregular refraction of the incident beam, resulting in malfunction of the light receiving element. Particularly, this problem will increase in the case that a large size prism is adapted to expand the region for detection of liquid adhered to the external surface of the wind-shield.